Decanter centrifuges include a rotating bowl in which a screw conveyor revolves at a slightly different speed.
Such centrifuges continuously receive and separate feed into layers of light and heavy phase materials which are discharged separately from the bowl. The screw conveyor moves the heavy phase material to a discharge port in a conical end portion of the bowl.
It is known from U.S. Pat. No. 3,795,361, assigned to the assignee of the present invention, that decanter centrifuges do not readily separate two materials of similar specific gravity, or if the heavy phase material is very slippery. Also, light and heavy phases having nearly the same specific gravity tend to re-mix when disturbed by the splashing introduction of feed or other turbulence.
U.S. Pat. No. 3,795,361 discloses centrifuge apparatus for efficient separation of feed mixtures which heretofore have defied separation. In such centrifuge apparatus, the weir surface of the heavy phase discharge port is at a greater radial distance from the bowl axis than is the weir surface of the light phase discharge port. In addition, a flat annular baffle and a feed cone is provided, both mounted coaxially on the hub of the screw conveyor for movement therewith.
The baffle of U.S. Pat. No. 3,795,361 is disposed normal to the bowl axis, preferably in position to partition the interior of the bowl into a separating zone and a discharge zone. The separating zone is surrounded by the cylindrical portion of the bowl; and the discharge zone is surrounded by the tapered end portion of the bowl. Being closely spaced from the inner surface of the bowl, the baffle defines a restricted annular passageway for the underflow of heavy phase material from the separating zone to the discharge zone. The flow area of this passageway is sufficiently large to prevent heavy phase material from accumulating in the separation zone, and desirably not larger than the solids discharge port. The baffle extends outwardly from the bowl axis beyond the interface between the separated phases in order to prevent the flow of light phase layer from the separating zone to the discharge zone.
The present invention is an improvement of the centrifuge construction disclosed in U.S. Pat. No. 3,795,361 and divisional U.S. Pat. No. 3,885,734, incorporated herein by reference, in that it seeks to further reduce turbulence and to present less restriction to the flow of heavy phase material through the space between the feed cone and the bowl, while retaining the improved separating efficiency for materials of similar specific gravity and slippery, fine solids. Furthermore, the present invention seeks to provide a screw conveyor which has a feed cone for admitting feed to the separating zone with minimum turbulence, and which also has a baffle that lends itself to easy change of its outside diameter. With an easy change of baffle outside diameter, the centrifuge may be adapted to changes of feed flow rate, solids concentration, and solids characteristics, whereby separation efficiency may be maximized.